The present invention relates to mutant transmembrane proteins which have increased conformational stability when compared to their parent protein, methods of selection and production. In particular the invention relates to mutant transmembrane proteins which are mutated in or in the proximity of the transmembrane alpha helices or in a kinked region or in an alpha-helix adjacent to a kink. The mutant transmembrane proteins have use in crystallization studies and also in screening to identify compounds for use in drug discovery and therapy.